FI57895B - FRONT PROCESSING FRAME RELEASE DIAPHRAGM - Google Patents

Description

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Patent- and regulatory registration 'An * ökw utla «d och utl. * Krlftt« i pubUcund 31.07.80 (32) (33) (31) fy ^ rtty stuolktes Bfirt prtartm 0U. 02.72

England-England (GB) 5351/72 (71) Imperial Chemical Industries Limited, Imperial Chemical House, Millbank,

London S.W.1, England-England (GB) (72) Christopher Vallance, Runcorn, Cheshire, England-England (GB) (7 * 0 Oy Kolster Ah (5 * 0 Method for making porous films - Forfarande för fram-ställning av porösa diäfragmor

The invention relates to a method for producing porous films. In particular, the invention relates to a process for the production of porous films based on polytetrafluoroethylene.

One method of making such porous films involves forming an aqueous slurry or dispersion of polytetrafluoroethylene and a solid particulate additive such as starch, adding an organic coagulant such as acetone to the dispersion, and then drying the coagulated dispersion. An organic lubricant such as petroleum ether is then added to the dried coagulated material to act as an excipient when the material is rolled into a film. When rolling is complete, the starch is removed to give the desired porous film. Lubricant can also be removed if necessary.

The main disadvantage of the above-mentioned method of manufacturing porous films is that the use of an organic lubricant increases the non-reproducibility of the films and this is particularly undesirable, especially when the films are to be used in multi-modular electrolytic cells where reproducibility is essential for efficient operation. It has now been found that the above-mentioned wound can be prevented 57895 or mitigated by the method of the present invention using water as a lubricant, which makes it possible to produce films with the desired reproducibility and permeability.

The invention relates to a process for preparing a porous film by preparing an aqueous slurry or dispersion comprising polytetrafluoroethylene and a solid particulate additive, thickening this aqueous slurry or dispersion to agglomerate the solid particles therein, forming at least 300 of the thickened slurry or dispersion preferably in the range of 1 x 10u to J x 10u, forming a film of the desired thickness from the doughy material and removing from the film a solid particulate additive characterized in that the doughy material contains sufficient water as a lubricant in the film-forming delivery and that thickening of the aqueous slurry reducing the water content of the slurry or dispersion and adding water to the thickened slurry or dispersion to form a dough-like material, or (b) placing the slurry or dispersion; dispersion under strong shear, or (c) by mixing the slurry or dispersion and adding a thickener.

Depending on the final use of the film, the desired degree of lubrication is achieved either by drying the aqueous slurry or dispersion to a low water content and then adding a substantial amount of water to form the dough or conversely drying the slurry or dispersion only slightly and adding relatively less water to form the dough.

When the films are to be used in electrolytic cells, the aqueous slurry or dispersion is preferably dried to a water content not exceeding 10% of the total weight of the dried dispersion.

In addition, water is preferably added to the dried dispersion until a dough with a water content of 2-50%, preferably 20-1 + 5% of the total weight of the dough is obtained.

Drying of the slurry or dispersion can be performed in any suitable manner that does not damage its components. Preferably, the drying is carried out at a temperature of 10 to 100 ° C, for example 15 to 50 ° C. The drying time depends on e.g. temperature, but is usually 10 to 100 hours, for example 20 to 50 hours.

In another embodiment of the invention, the aqueous slurry or dispersion is thickened by subjecting it to a strong shear effect and this strong shear effect is maintained until a dough having a viscosity of 3,57895 of at least 300 poise, preferably 1 x 10 x to 7 x 10 10 poise is obtained.

In this embodiment, the water content of the slurry or dispersion is preferably 2-50%, in particular 20-50% of the total weight of the dispersion.

A particularly suitable way of subjecting a slurry or dispersion to high shear conditions is to mix the slurry or dispersion in a Z-blade mixer.

When the slurry or dispersion is subjected to high shear conditions, the viscosity increases and the strong shear effect is maintained at least until the viscosity reaches the desired range.

In another embodiment of the invention, the aqueous slurry or dispersion is thickened by first subjecting it to a mixing effect and then adding a thickener to achieve the desired viscosity for film formation.

Preferably, the thickener is a copolymer of maleic anhydride and alkyl vinyl ether.

The particle size of the polytetrafluoroethylene in the aqueous slurry or dispersion is preferably in the range of 0.05 to 1, for example 0.1 to 0.2.

The solid particulate additive may be any substance which is substantially insoluble in water but which can be removed by suitable chemical or physical means which do not damage the polytetrafluoroethylene. The additive may be a starch, for example maize starch and / or potato starch, or a water-insoluble inorganic base or carbonate, for example calcium carbonate.

Additives can be removed, for example, by dissolving the film in an acid, preferably a mineral acid, e.g. hydrochloric acid. Other additives that can be used are organic polymers, which, depending on the properties of the polymer, can be removed from the film by dissolving in an organic solvent, hydrolyzing or evaporating. Mixtures of additives can be used and, if necessary, the films can be treated in several ways to remove the additive.

In general, the particle size of the additive is substantially entirely between 5 and 100. The amount of additive depends on the desired permeability of the final film. Thus, the weight ratio of additive to polytetrafluoroethylene may be, for example, between 10: 1 and 1:10, preferably between 5: 1 and 1: 1.

In many cases, it is desirable to mix other components into the aqueous slurry or dispersion that are not removed when the film is treated to remove the particulate additive. Examples of such components are particulate fillers, generally inorganic fillers, for example titanium dioxide, which is particularly preferred, barium sulphate, asbestos (e.g. amphibole or serpentine asbestos), graphite and alumina. The particle size of the filler is preferably, for example, less than 10 [mu] m and particularly preferably less than 1 [mu] m. The weight ratio of filler to polytetrafluoroethylene may be, for example, between 10: 1 and 1:10, preferably between 2: 1 and 1: 2.

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In some cases, it may be advantageous to add a coagulant, e.g., saline, to the dispersion to aid in dough formation.

The film is usually formed from the dough by calendering. Preferably, the calendering is performed by feeding the dough through the rolls numerous times.

Generally, after a few or even each pass through the calender rolls, the film sheets are rotated about 90 ° so that the calendering takes place biaxially.

The films prepared according to the process of the present invention have a wide range of applications, being particularly suitable for use in electrolytic cells for the electrolysis of alkali metal halides to produce chlorine and alkali hydroxide lyes.

They are generally strong enough to be used without support, but for additional strength, it is advisable to mix a suitable reinforcing material in the film, for example a polymeric gauze such as polypropylene gauze.

The method of the present invention makes it possible to easily produce films from time to time in succession, all of which have the same permeabilities. This is very important when membranes are used in electrolytic cells.

The invention is further illustrated by the following examples in which all parts and percentages are by weight.

Example 1 To 100 parts of an aqueous dispersion of polytetrafluoroethylene containing 60% of polymer in the form of particles, almost all of which ranged in size from 0.15 to 0.2 .mu.m, were added 101 parts of water, 60 parts of titanium dioxide having a particle size of about 0.2 .mu.m. 60 parts of corn starch having a particle size of about 13 μm and 120 parts of potato starch having a particle size of less than 75 μm. The mixture was then stirred with a paddle stirrer for 30 minutes to make a substantially uniform paste. The paste was applied to troughs and dried at 21 ° C for 1 * 8 hours to a water content of 5 x 7% by weight. 100 parts of the obtained crumbs were mixed with 52 parts of water to form a dough with a viscosity of U x 10. The dough was then applied to the shorter edge of a rectangular piece of paperboard and then calendered onto the paperboard between two equally fast calender rolls into an elongated film with a gap between the rolls of 3 mm. After calendering, the elongate film was cut in four equal parts in the calendering direction. These were uniformly superimposed on a four-layer laminate. The board was lifted, rotated 90 ° horizontally, and calendered (at a 90 ° angle to the original fish direction) through a 3 mm roll gap. This delivery, followed by cutting into four sections, overlapping, inversion, and calendering, was repeated until the combination was calendered a total of five times. The resulting laminate was cut into four parts in the calendering direction, stacked, removed from the paperboard, and calendered without turning 90 °, whereby the gap between the rolls 5,57895 was reduced by the thickness of the paperboard. After calendering, the laminate was cut, from a rectangle in the calendering direction, into four equal parts, the parts were stacked on top of each other, turned 90 ° and calendered again. This delivery, cutting from the rectangular calendering direction, stacking, inverting, and calendering was repeated until the combination was calendered a total of 9 times. The resulting substantially rectangular laminate was then fed through the rollers with a longer side at a 90 ° angle to the calendering direction and a slightly narrowed slit without cutting, stacking or turning 90 °. This delivery was repeated through a gradually reduced gap between the rolls, feeding the laminate to the rolls each time with the same edge first until the thickness of the laminate was 1.5 mm. A rectangular 22 x 26 mesh gauze woven from 0.279 mm diameter monofilament polypropylene yarn was placed on the surface of the laminate and rolled into the laminate by calendering through a slightly reduced gap between the rolls. The resulting reinforced film was removed from the rolls and soaked in cold aqueous $ 18 hydrochloric acid for 2 hours. The starch additive was then removed to give a highly porous film suitable for use as a diaphragm in electrolytic cells in which aqueous solutions are electrolyzed.

Example 2 To 100 parts of an aqueous dispersion of polytetrafluoroethylene containing 60% polymer in the form of particles, almost all of which ranged in size from 0.15 to 2 .mu.m, 70 parts of water, 60 parts of titanium dioxide having a particle size of about 0.2 .mu.m, 60 parts of corn having a particle size of about 13 and 120 parts of potato starch having a particle size of less than 75. The mixture was then mixed with a paddle mixer for 3 minutes to form a paste, which was then mixed in a Z-blade mixer for 22 minutes to form a dough with a viscosity of k x 10. The dough was then applied to the shorter edge of a rectangular piece of paperboard and calendered onto the paperboard between two equally rapidly rotating rolls into an elongated film with a gap between the rolls of 3 mm. After the fish terrier, the elongate film was cut in four equal parts in the calendering direction. These were uniformly superimposed into a four-layer laminate. The board was lifted, rotated 90 ° horizontally, and calendered (at a 90 ° angle to the original calendering direction) again through a 3 mm roll slot. This delivery, followed by cutting into four parts, stacking, turning, and calendering, was repeated until the combination was calendered a total of 15 times. The resulting laminate was cut into four parts in the calendering direction, the parts were stacked on top of each other, removed from the board and calendered without turning 90 °, whereby the gap between the rolls was reduced by the thickness of the board. After calendering, the laminate was cut, from a rectangle in the calendering direction, into four equal parts, the parts were stacked on top of each other, turned 90 ° and calendered again. This delivery, cutting from a rectangular calendering direction, stacking, turning, and calendering was repeated until 6,57895 combinations were calendered a total of nine times. The resulting substantially rectangular laminate was then fed through the rollers at a longer side at a 90 ° angle to the calendering direction, without cutting a slightly narrowed slit, stacking on top of or turning 90 °. This delivery was repeated through a gradually reduced gap between the rolls, feeding the laminate to the rolls each time with the same edge first until the thickness of the laminate was 1.5 mm. A rectangular 22 x 26 mesh gauze woven from 0.279 mm diameter monofilament polypropylene yarn was placed on the surface of the laminate and rolled into the laminate by calendering through a slightly reduced gap between the rolls. The obtained reinforced film was removed from the rolls and dissolved in cold aqueous 18% hydrochloric acid for 2 hours. The starch additive was then removed to give a highly porous film suitable for use as a diaphragm in electrolysis cells in which aqueous solutions are electrolysed.

Example 3 To 100 parts of an aqueous dispersion of polytetrafluoroethylene containing 60% of polymer in the form of particles, almost all of which ranged in size from 0.15 to 0.2 μm, were added 200 parts of water, 60 parts of titanium dioxide having a particle size of about 0.2 μm. 60 parts of corn starch having a particle size of about 13 μm and 120 parts of potato starch having a particle size of less than 75 μm. The mixture was then stirred with a paddle mixer for 2 minutes to form a substantially smooth paste with a viscosity of 3 poise. To this paste was added 27 parts of a thickener known as Viscofas L 100 (a copolymer of maleic anhydride and methyl vinyl ether), and 27 parts of 1M NaOH and the mixture were mixed with a paddle mixer to give a dough with a viscosity of 1 x 10. The dough was then applied to the shorter edge of a rectangular piece of paperboard and calendered onto the paperboard between two equally fast calender rolls into an elongated film with a gap of 3 mm between the rolls. After calendering, the elongate film was cut in four equal parts in the calendering direction. These were uniformly superimposed on a four-layer laminate. The board was lifted, rotated 90 ° horizontally, and calendered (at a 90 ° angle to the original calendering direction) through a 3 mm gap. This delivery, followed by cutting into four parts, stacking, turning, and calendering, was repeated until the combination was calendered a total of 110 times. In the first 90 runs through the rollers, precise stacking into laminates was not possible due to the nature of the material. The obtained laminate was cut into four parts rectangularly in the calendering direction, the parts were stacked on top of each other, removed from the board, turned 90 ° and calendered, whereby the gap between the rolls was reduced by the thickness of the board. This delivery, cutting at right angles in the calendering direction, stacking, turning, and calendering, was repeated until the combination was calendered a total of 115 times.

τ 57895

The resulting substantially rectangular laminate was then fed through the rollers at a longer side at a 90 ° angle to the calendering direction, without cutting a slightly narrowed slit, stacking on top of or turning 90 °. This delivery was repeated through a gradually reduced gap between the rolls, feeding the laminate to the rolls each time with the same edge first until the thickness of the laminate was 1.5 mm. A rectangular 22 x 26 mesh gauze woven from 0.279 mm diameter monofilament polypropylene yarn was placed on the surface of the laminate and rolled into the laminate by calendering through a slightly reduced gap between the rolls. The resulting reinforced film was removed from the rolls and soaked in cold aqueous $ 18 hydrochloric acid for 2 hours. The starch additive was then removed to give a highly porous film suitable for use as a diaphragm in electrolytic cells in which aqueous solutions are electrolyzed.

In the process of the invention, high-viscosity measurements were performed on a Weissen-Berg rheogonometer, Model R16 (Sangamo Controls Limited). This device is equipped with a cone and a plate-sample unit with a diameter of 2.5 cm and a cone angle U °. The cone and plate are made of stainless steel and their. ... 3. . . . . ........

the surfaces are gilded. For viscosities less than 5 x 10, torque rod torque measuring accessories were used and the cone was rotated continuously. 3 ..... ...

paan. For viscosities greater than 5 x 10, the cone was rotated in an oscillating manner and torque was measured using two Type 9203 piezoelectric transmitters (Kistler Instruments Limited) spaced 1+ cm apart. The obtained output signal was compared with the input signal using analyzers JM1660 and JX1600A (Solatron Limited). Detailed viscosity measurements are initial readings at a shear rate of y = 1 sec. \

However, for viscosity measurements below 600, for example, as in the first viscosity measurement of Example 3, a Brookfield Viscometer, model LVF meter was used. This device was equipped with a rotor number 2 and said measurements were initial measurements taken at speed 30.

Tests have also been performed to show that films prepared according to the methods of this invention can be obtained with a higher degree of reproducibility than films obtained with organic lubricants.

Test series A

A polytetrafluoroethylene film was prepared according to British Patent No. 1.01.0, 0i + 6 using acetone as a coagulant and petroleum ether as a lubricant. Permeability tests were performed on these membranes and the results are shown in Table 1 below. Measurements were made on membranes mounted on electrolytic cells.

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Table I

Membrane No. K _ Perm 1 8350 2 8Uo K Perm, (permeability per hour, h *) flow rate through the membrane (liters / h) x 10 ^ ____ “λ membrane area (cm) x pressure drop (cm, saline)

In Test Series A, the membrane was mounted on an electrolytic cell and the inorganic filler was removed using the cell. The permeability of the film is initially zero, it increases as the starch is removed and decreases as the film becomes clogged during prolonged use of the cell. The permeability coefficient (K Perm) is based on measurements taken at maximum membrane permeability. A comparison of the permeability coefficients shown in Table I for films prepared by the same method showed a wide deviation in permeability.

Test series B

In this series of tests, a number of films were prepared according to the method described in Example 1, except that petroleum ether was used as a lubricant instead of water. In this case, the inorganic filler was extracted outside the electrolysis cell by soaking in a 16 gun in hydrochloric acid, and permeability measurements were made after the membranes were installed in the electrolysis cells. The results are shown in Table II below.

Table II

Film No. K Perm.

3 UU6 U 765 5 b92 6 UU6 7 U90 8 253 9 612 10 321 11 262 12 20b 13 292 ib 959

Average K Perm * U62

Mean deviation = 226 9 57895 These results show that the use of an organic lubricant significantly increases the non-reproducibility of the films.

Test series C

In this series of tests, films were prepared according to Example 1 using water as a lubricant. This time, a slightly different method was used to measure the permeability by providing a special device for removing the inorganic filler by acid extraction and performing permeability measurements in this special device and not with the membranes installed in the electrolysis cell. The test results are shown in the following Table III, which also shows the results of tensile strength tests obtained on films with a standard Instron tensile strength measuring device. It has been found that the tensile strength of the film is proportional to its permeability, and it has been found that tensile strength tests are easier to perform and that the results can be used to accurately predict the permeability of the film.

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Table 111

Kalvo at o K pan. x 10 ^ Final tensile strength (gm mm “2) 15 32 37.2 16 30 40.7 17 20 43.6 18 30 40.0 19 32 39.3 20 32 42.9 21 36 46.0 22 30 37.0 23 28 44.0 24 24 40.0 25 34 40.0 26 34 39.4 27 26 39.2 28. 32 .49.0 29 20 42.0 30 20 46.0 31 20 45.8 32 20 45.7 33 20 46.0 i, 34 21 47.8

Average - 27 * 1 Average - 42.6

Mean deviation «5 * 8 Mean deviation - 5.6 j

The results summarized in Table III show that »using water as a lubricant» can be used to obtain films »with the desired repeatability and permeability ·

Claims (1)

"57895 Claim: A method of making a porous film by preparing an aqueous slurry or dispersion comprising dust tetrafluoroethylene and a solid particulate additive, thickening the aqueous slurry or dispersion to agglomerate at least 300 solids and a dispersion of solid particles therein, forming a thickened slurry or dispersion; in the range of 1x10 to 7 x 10 6 by forming a film of the desired thickness from the dough-like material and removing a solid particulate additive from the film, characterized in that the dough material contains sufficient water as a lubricant in the film-forming delivery and that thickening of the aqueous slurry or dispersion ) by reducing the water content of the slurry or dispersion and adding water to the thickened slurry or dispersion to form a dough-like material, or (b) placing the slurry or dispersion the dispersion under strong shear, or (c) by mixing the slurry or dispersion and adding a thickener.